1. Field of the Invention
The present invention relates to a tuning control apparatus of a receiver. More specifically, the present invention relates to a tuning control apparatus in a receiver adapted for electronically controlling a direct current voltage applied to a variable capacitance diode included in a tuner that operates based on a digital numerical value associated with a broadcasting frequency.
2. Description of the Prior Art
A tuning control system of a receiver employing a voltage controlled oscillator serving as a local oscillator in a tuner may be classified into two types, as well known in the art. One is a frequency synthesizer type employing a phase locked loop and the other is a voltage synthesizer type employing digital/analog conversion. A tuning control system employing a frequency synthesizer is disclosed in, for example, U.S. Pat. No. 4,081,752, issued Mar. 28, 1978 to Yasuaki Sumi. Such a frequency synthesizer type is adapted such that an oscillation output obtained from a crystal controlled oscillator is used as a reference frequency and a tuning voltage is generated responsive to a phase difference between the reference frequency and the local oscillation frequency. Accordingly, the accuracy and stability of the tuning are excellent. However, such a type of tuning is liable to be influenced by a harmonic component generated by a frequency divider included in a phase locked loop, a fluctuation of a phase locked loop, or the like. Therefore, the noise characteristic of the tuner as a whole may be degraded. In the case where a ceramic filter is employed in an intermediate frequency stage, another disadvantage is encountered in that the tuning accuracy is influenced by the diversity of the characteristic of such a ceramic filter.
On the other hand, a voltage synthesizer type is disclosed in, for example, U.S. Pat. No. 3,968,440, issued July 6, 1976 to George John Ehni, III. Since a tuning control system employing such a voltage synthesizer utilizes a direct current voltage obtained from a digital/analog converter as a tuning voltage, an advantage is brought about in that a high signal-to-noise ratio can be attained using a conventional tuner design technology. Nevertheless, a tuning control system of the voltage synthesizer type involves a problem in conjunction with tuning accuracy and stability.